a) Field of the Invention
The invention is directed to an arrangement for the optoelectronic recording of large-area fingerprints, particularly for acquiring prints of the entire palm of the hand. It is used for the recording of personal features for identification services and forensic identification.
b) Description of the Related Art
In law enforcement identification services, fingerprints and handprints of individuals are taken in order to be able to establish their identity. For more than one hundred years, the recording of the patterns of the papillary ridges on the finger and hand has conventionally been carried out by inking the surfaces of the finger or hand and subsequently printing them on paper. With the availability of high-performance electronic sensors and computers, arrangements have become known which are used for direct electronic acquisition of the patterns without the intermediary of ink, paper and scanning of the print image. In this way, the electronic acquisition can be carried out substantially faster and, above all, with higher quality.
While there have been many diverse publications concerned with the electronic acquisition of prints of individual fingers (more exactly, the upper two joints of the finger), only few solutions are known for acquiring the palm of the hand. These solutions are directed to arrangements which are either limited in format and allow only part of the palm to be acquired in one process or which can acquire the entire full-size hand but do not reliably achieve the quality necessary for applications of this kind.
The pre-published application US 2002/0090147 A1 describes an arrangement for optically recording the patterns of the palm of a hand in which the pattern of the papillary ridges lying on a plane recording surface of a prism is rendered visible as an image by means of frustrated total internal reflection (FTIR). For this purpose, the prism which is made of an optical solid material (e.g., glass) is supported in a stationary manner and incorporated in an optical imaging beam path for imaging the hand supporting surface on an image sensor. A mirror which can be tilted in two axes in a controlled manner is located in the beam path to increase the resolution of the two-dimensional image recording sensor through a special method with multiple recording of the images and subsequent combination of the image data.
In this arrangement, it works out disadvantageously that the palm of the hand is more or less curved inwardly and for this reason does not make direct contact in its entirety with the recording surface so that relatively large areas (usually in the center of the palm) are not imaged (i.e., appear white or empty). A practical remedy can be to apply pressure to the top of the hand resting upon the recording surface. However, given the concave surface of the hand, this improves the imaging only to a certain extent because the increased contact pressure on the parts of the hand that already had good contact before presses the papillary ridges together and the pressed areas in the resulting image appear very dark with poor contrast and are therefore more difficult to evaluate.
U.S. Pat. No. 6,038,332 describes an arrangement for the optical recording of patterns on the palm of a hand. The curved surface of a transparent half-tube (a glass tube that is split along its length) serves as a stationary support for the palm. In the interior, it is ensured by means of a light rod which extends linearly along the curved surface and which illuminates the curved surface obliquely that only light which is scattered at the outer surface of the hand resting on the support is coupled into an optical imaging beam path by a mirror. The imaging of the palm of the hand in its second dimension is generated by motor-actuated rotation of the mirror and by optics on an image sensor which is essentially one-dimensional, read out in the form of (overlapping) partial images and put together to form the output image.
It is disadvantageous that the papillary ridges are made visible and imaged using the principle of scattered light because residues from perspiration excreted by the hand must be meticulously removed in their entirety after every placement of the hand so that scattered light from old prints is not detected again.
U.S. Pat. No. 6,175,407 also discloses an arrangement for the optical recording of the papillary ridge patterns on the palm of a hand in which a cylinder of solid optical material (e.g., glass) is rotatably supported, prisms being arranged near the end faces of this cylinder in order to radiate illumination light at a flat angle to the outer surface of the solid cylinder and couple out illumination light on the opposite side in an optical imaging beam path.
The outer surface of the cylinder serves as a support for a portion of the palm that is placed thereon. By means of this arrangement, the image of the supported papillary ridges is made visible portion by portion using the principle of frustrated total internal reflection (FTIR) and imaged by optics on a one-dimensional image sensor. The second image dimension is scanned by rotating the solid cylinder by means of the hand and is assembled line by line to generate the output image. However, when the hand is moved too quickly over the cylindrical surface, i.e., when the angular velocity exceeds a maximum permissible velocity, the signal processing means of the image sensor are incapable of supplying image data so as to be synchronized with the rotating movement and, consequently, the output image is also not assembled correctly. For this reason, very complex additional arrangements are provided for measuring the angular velocity and for braking the cylinder when its given rotational speed is exceeded.
Another basic problem consists in that the palm must be moved exactly without relative movement between the palm and the cylinder surface in order to avoid drastic errors in assembling the output image. With regard to practical realization further disadvantages consist in that there are considerable difficulties in sealing the optical light paths at both end faces of the rotatable cylinder in the transition to the prisms for preventing malfunction due to dust and moisture.
Further, another prior art arrangement for optical recording of the patterns of palms according to publication US 2004/0109245 uses a conical optical body. The principle of frustrated total internal reflection (FTIR) is again used in that a rod with line-shaped illumination is arranged in a bore hole along the axis of symmetry of the cone and the area of the conical outer surface which is accordingly illuminated virtually in a line-shaped manner is imaged downward on a line sensor through the planar base surface of the cone by imaging optics. The sensor and the optics are rigidly coupled to a rotatable plate which, further, is fixedly connected to the rod of the line-shaped illumination and all three components are moved jointly around the axis of symmetry of the cone. In this way, the outer surface is scanned line by line along the surface lines of the cone and an image of the palm that is placed thereupon one time is outputted as a quantity of individual lines corresponding to the outer surface of the cone that is unwound in a plane. However, in order to convert the image to a rectangular format, a coordinate transformation and resampling of the entire image must be carried out. While this operation is uniquely determined and is relatively simple to describe mathematically, it necessarily leads to contrast gradients in the effective optical modulation transfer function (MTF). Therefore, the MTF must deliver a substantially higher contrast in the raw image than is required (after transformation) in the output image according to relevant standards. Accordingly, the technical requirements for optics and sensor are appreciably higher than in an arrangement using the cylinder principle. This means that technical expenditure is considerably higher; otherwise, only a lower image quality (resolution in dpi) can be realized in the output image.